The invention was originally conceived as a method for repairing steel pipe lines. While not limited to that application, it will now be discussed in connection with that specific application.
Oil and natural gas transmission pipe lines commonly develop defects in the pipe wall, usually in its outer surface. Predominantly, these defects take the form of corrosion pitting, which is initiated when the protective coating on the pipe is damaged and moisture comes into contact with the steel surface of the pipe. The resulting galvanic corrosion cell develops a string of rusty pits of varying depth. In addition to corrosion pitting, there can also be other defects such as dents, gouges, grooves, and cracks, which arise as a result of poor installation practises, third party acts, or metal fatigue during service.
In cases where the defect is shallow in nature, it is usual to repair the existing pipe rather than to cut out and replace the damaged segment.
The type of repair applied to a shallow defect depends on factors such as its orientation, size and depth, the pipe wall thickness, and the maximum operating pressure.
If the corrosion is not very extensive, the corrosion product is simply cleaned out and the pits are filled with hardenable mastic or epoxy, to restore the smooth pipe contour. The pipe segment is then re-wrapped with new protective coating and placed back in service.
If the defect is more serious, it is common practice to apply a full encirclement sleeve to engirdle the defect and reinforce the pipe at the locus of the defect. The present invention is concerned with this sleeve type of repair.
The sleeve repair procedure commences with some of the same steps as those used with the minor defects. That is, the protective coating is removed from the pipe segment to be repaired, the dirt and corrosion products are cleaned out of the pits, and the pits are filled with mastic or epoxy. Then a steel sleeve, split longitudinally into two half shells, is placed around the pipe over the defect. The two half shells are welded together by a longitudinal butt weld formed along one pair of adjacent ends, with the shells in a relaxed condition. A chain or cable is then tightened around the exterior of the sleeve using a ratchet tensioner, to clamp the sleeve tightly to the pipe. When the shells are clamped in this manner, a slight longitudinal gap still exists between the two remaining free ends of the sleeve. A second longitudinal butt weld is then applied along this gap, to join the free ends and convert the sleeve into a closed collar.
If there is concern that a leak will subsequently develop at the defect, it is common practice to apply a circumferential fillet weld between the pipe and the sleeve at each end of the sleeve. In this manner, a pressure tight vessel is created around the defect, to contain the leak should it develop.
To execute the circumferential fillet welds, it is necessary to heat the pipe and the sleeve to the proper welding temperature along the area to be welded. In the case of liquid transmission pipe lines, the presence of stationary or flowing liquids significantly affects the cooling rate of circumferential fillet welds. Similarly, in the case of a gas line, the segment being welded must be filled with water or the like, to avoid the possibility of an explosion. The presence of the liquid accelerates the cooling rate. When the cooling rate is too fast, the pipe steel has a tendency to develop martensitic microstructures (hard spots) along the toe of the circumferential fillet weld. This condition, known as embrittlement, particularly manifests itself in the older pipelines now undergoing repairs which are known to have a higher carbon content than modern pipe steels.
Eventually, these welds can fail catastrophically by cracking when they are subjected to severe external stresses imposed by a variety of soil conditions, such as poor backfill compaction following sleeve installations, soil movement from freeze-thaw cycles, and insufficient cover under travelled areas.
This problem with circumferential sleeve welding is such that many pipe line companies do not allow this type of sleeve repair to be made on their pipe lines, preferring to cut out and replace the damaged segment instead.
To prevent a feeling to the reader for the high incidence of these repairs, a pipe line company in Alberta recently replaced a 1/2 mile section of pipe that had 16 sleeve repairs in place along its length. However, in balance it needs to be said that there are other long segments of line that are free of sleeve repairs.
In summary, the repairs to be made are numerous, they have involved for decades the use of the repair sleeve procedure previously described, and the problem connected with the circumferential fillet weld has not heretofore been solved.